Correlating structural distributions in silica glass with two-dimensional J-resolved spectroscopy

Article

关键词: QUADRUPOLAR COUPLING PARAMETERS;    BOND-ANGLE DISTRIBUTION;    NUCLEAR-MAGNETIC-RESONANCE;    SI-29 NMR-SPECTROSCOPY;    VITREOUS SILICA;    MAGIC-ANGLE;    O-17 NMR;    NEUTRON-SCATTERING;    HIGH-RESOLUTION;    SPIN-ECHO;   

DOI  :  10.1103/PhysRevB.98.134202

来源: SCIE

【 摘 要 】

A two-dimensional (2D) J-resolved magic-angle spinning nuclear magnetic resonance (NMR) spectrum of silica glass at Si-29 natural abundance levels, 4.7%, was measured using the shifted-echo phase-incremented echo train acquisition (SE-PIETA) pulse sequence. At Si-29 natural abundance levels the J(Si-O-Si) coupling splittings appear as overlapping doublet patterns arising from isolated Si-29-O-Si-29 linkages. The experimental 2D J-resolved spectrum is analyzed to obtain a bivariate probability distribution correlating the central Si-O-Si angle of a Q(4)-Q(4) linkage to its mean Si-O-Si angle (seven angles) using relationships between Si-29 isotropic chemical shifts and geminal J(Si-O-Si) coupling of a Q(4)-Q(4) to its local structure. To obtain a self-consistent bivariate probability distribution it was necessary to introduce an additional dependence of the Si-29 chemical shift of a Q(4) on mean Si-O distance as well as mean Si-O-Si angle. The implication of this necessary modification is a positive correlation between Si-O-Si angle and Si-O distance in the silica glass, consistent with recent O-17 NMR measurements on ambient and densified silica glasses but running opposite to the trend generally found in crystalline silica polymorphs. From the analysis of the Si-29 2D J-resolved spectrum we determine a Si-O-Si bond angle distribution in silica glass as having a mean at 147.8 degrees, a mode at 147 degrees, and a standard deviation of 10.7 degrees. Our statistical model for analyzing the experimental Si-29 2D J-resolved spectrum also indicates that the individual Si-O-Si bond angle distributions are relatively uncorrelated.

【 授权许可】

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